1. Field of the Invention
The present invention relates to installation of electrically heated subsea pipelines. More particularly the invention relates to methods for installation of electrically heated subsea pipelines having a pipe-in-pipe configuration.
2. Description of Related Art
Offshore hydrocarbon recovery operations are increasingly moving into deeper water and more remote locations. Often satellite wells are completed at the sea floor and are tied to remote platforms or other facilities through extended subsea pipelines. Some of these pipelines extend through water that is thousands of feet deep, where temperatures of the water near the sea floor are in the range of 40xc2x0 F. The hydrocarbon fluids, usually produced along with some water, reach the sea floor at much higher temperatures, characteristic of depths thousands of feet below the sea floor. When the hydrocarbon fluids and any water present begin to cool, phenomena occur that may significantly affect flow of the fluids through the pipelines. Some crude oils become very viscous or deposit paraffin when the temperature of the oil drops, making the oil practically not flowable. Hydrocarbon gas under pressure combines with water at reduced temperatures to form a solid material, called a xe2x80x9chydrate.xe2x80x9d Hydrates can plug pipelines and the plugs are very difficult to remove. In deep water, conventional methods of depressurizing the flow line to remove a hydrate plug may not be effective. Higher pressures in the line and uneven sea floor topography require excessive time and may create operational problems and be costly in terms of lost production.
The problem of lower temperatures in pipelines has been addressed by a variety of heating methods, including electrical heating. Most of the proposals for electrical heating of pipelines have related to pipelines on land, but in recent years industry has investigated a variety of methods for electrical heating of subsea pipelines. (xe2x80x9cDirect Impedance Heating of Deepwater Flowlines,xe2x80x9d OTC 11037, May, 1999). One electrical heating method is the pipe-in-pipe method. In one configuration of a pipeline using this method, a pipe-in-pipe subsea pipeline is provided by which a flow line for transporting well fluids is surrounded concentrically by and electrically insulated from an electrically conductive outer pipe until the two pipes are electrically connected at the distal or remote end of a heated segment by a bulkhead. Voltage is applied between the inner and outer pipes at the proximate or electrical input end and electrical current flows along the exterior surface of the inner pipe and along the interior surface of the outer pipe. This pipe-in-pipe method of heating is disclosed, for example, in U.S. Pat. No. 6,142,707. Other variations of the general pipe-in-pipe method exist. The electrical power is supplied through an electrical isolating joint at the power input end of a segment of line to be heated. Alternating current, normally at about 60 Hz, is used. The voltage across the annulus is highest at the isolating joint and falls linearly to zero at the bulkhead. The current is essentially constant along the entire length of the pipe segment that is heated. Two key electrical effects, the skin effect and the proximity effect, confine the current flow largely to the annulus surfaces. Consequently, most of the current is effectively isolated from the produced fluids and the seawater around the pipeline.
In pipe-in-pipe electric heating configurations, annulus apparatus that electrically isolates the inner and outer pipe and provides thermal insulation and load sharing is desirable. Electrical isolation between the inner and outer pipe is needed so that the pipes will not short out. Thermal insulation is advantageous because it minimizes heat loss from the inner pipe and reduces the amount of electrical current necessary to achieve the desired temperature in the inner pipe. Load sharing between the pipes helps limit the stress on the outer pipe during laying.
A method for installation of apparatus in the annulus that prevents complete flooding of the annulus and confines flooding caused by a breach of the outer pipe to a subsection of the annulus is needed. There is also a need for methods for installation of apparatus that allows for electrical heating in the presence of small amounts of water in the annulus.
Towards providing these and other advantages, the present invention provides a method for installing a segment of an electrically heated pipe-in-pipe subsea pipeline by spraying on an insulating ring along the outer surface of inner pipe joints, inserting the inner pipe joints in outer pipe to form a pipe-in-pipe segment, placing the pipe-in-pipe segment at a selected angle on a pipe-laying barge, inserting water seals at selected locations in the annulus and joining the segment to another segment of the pipeline that has previously been placed subsea. A method for forming a pipe-in-pipe quad is provided. Double joints are formed and joined to form quad of both the inside and outside pipe. Double joints of the inside pipe are insulated by a spraying process and joined by welding and the weld-affected area is then insulated and protected from risk of water in the annulus causing an electrical short. A method for forming a water stop in an annulus using a liquid polymer that solidifies in the annulus is also provided.